The Darkness is a mysterious ongoing cosmological phenomenon plaguing the world during the events of The Banner Saga.
Though mentioned in the first game, the Darkness does not make itself apparent until The Banner Saga 2. It acts as a massive sphere of dark energy, slowly engulfing the known world from the Northeast and consuming everything it touches. This Darkness is the cause of the massive migration of the Dredge from the northern regions of Valkajokull to the south/south-west, and soon causes most of the surviving human and varl residents to flee to Arberrang. It is the source of Warped enemies, which are a major obstacle in The Banner Saga 3.
The Darkness is a byproduct of the Dark Sun, an alternate version of the Sun which gives light to Inner Earth, the home of the Dredge.
Before the events of The Banner Saga, Juno was executed by the Council of Menders for committing their gravest crime: altering the mind of her lover Eyvind in an attempt to cure his insanity. Exiled from the Council and beset by grief, Eyvind journeyed to the White Tower in Inner Earth, where he siphoned the power of the Dark Sun to resurrect Juno. His actions caused the Dark Sun to crash into the Earth, releasing the spread of Darkness -- and prematurely hatching the world-eating Serpent.
"I expected desolation. Instead, we've passed villages that look as though melted, and forests curled like fish hooks. I have a bad feeling that these things will never again return to the way they once were, even if we succeed." -Iver, The Banner Saga 3
The Darkness does not destroy the landscape it consumes; instead, it changes it, creating a monstrous version of what once was. Plant life is grotesquely mutated, shifting to a mixture of blues, blacks and purples, while stone and wood are twisted beyond recognition. The terrain itself also seems to change, becoming uneven and jagged.
Most notably, any living being consumed by the Darkness -- whether they be human, varl, horseborn or dredge -- is changed into a Warped creature. Warped creatures undergo physical mutations, and their minds are reduced to those of intelligent, yet savage, beasts.
The Banner Saga
"I am the end. Do you understand? This world, and this tapestry, I would devour. It is my purpose. But I cannot. Instead, now comes a wall of night to consume your pitiful world." -The Serpent, The Banner Saga
Little is initially known about the Darkness in the first part of The Banner Saga. It is first mentioned by the Serpent to Juno in Chapter 5, and the characters correctly theorize that it is the reason for the massive dredge invasion.
The Banner Saga 2
The Darkness is first sighted at the end of Chapter 11, as the Ravens and Zefr emerge from the mines beneath Bindal. It can be seen in the distance as the Ravens make their way to Manaharr, coming across increasingly-thick hordes of fleeing dredge on the way.
The Banner Saga 3
In the third part of The Banner Saga, Juno, Eyvind, Iver and the Ravens journey through the Darkness, utilizing a sphere of light to protect them from its warping effects. Juno and Eyvind have a plan to stop the Darkness, though they keep it secret from the rest of their party for much of the journey; eventually, they reveal to the rest of the party that the Darkness was released on to the world as a consequence of Eyvind utilizing the Dark Sun to resurrect Juno.
The party wanders across the corrupted landscape to the Inner Earth, where they make their way to the White Tower, the only place where they can stop the spread of the Darkness. On the way, they are forced to fight hordes of Warped creatures, and are pursued by Bolverk, who was possessed by the Serpent during the collapse of Manaharr.